Yes if you're gonna pull way to much current through the cable the modular connector will melt first.
and whether it is 18AWG or 16AWG only affects efficiency but not enough to justify the price differential.
Not only efficiency. It will cost efficiency because you get voltage drop in the wires, that's the reason of the loss of efficiency. The more current you pull through a wire how higher the resistance and therefore the bigger the voltage drop gets. However because most modern power supplies, including Seasonic Focus, use feedback wires which measure the voltage on the end of the wire to compensate this drop in voltage.
But because these feedback wires are implemented in the 24 pins ATX connector in (almost) all power it will only compensate voltage drop in the 24 pins ATX kabel. If the voltage drop is small in the 24 pins ATX cable because of a low load. It could mean you have a big voltage drop on the end of PEG cable if you pull much current through that cable when you use thin wires.

Simple schematic about this.
It also has effect on transient response. Because the wire/connector will resist more with higher current. This is something much more difficult to compensate. It's probably not even good possible, because the PSU can't respond that fast. Because of that it's better to not pull to much current. No one is testing this by the way. Aris does transient response tests, but those are with much lower current, no where close to the spikes a modern gaming PC can go. The reason that there weren't any issues with the Focus series in the reviews.
We use high-current terminal connectors so together with the 75W (not sure if this still holds true) supplied by the PCIe slot on the MB to the VGA; the PCIe cable, be it with a single connector or double connectors daisy chained, can support a VGA card up to 380W.
The max a PCI Express card can take through the slot is indeed still 75W. But in case of a high-end graphics card it's mostly around the 30W under load.
So what changed from the 225W from before to the 380W? This also means that cables/connectors from before aren't compatible with the current cables/connectors?